Today, after extensive thinning of my plants, I managed to get a reading with the PAR meter, at the substrate, about 8" to the left of the centerline and at about the centerline from front to back. The reading was 50 - 80, depending on whether I moved an inch one way or another. This is reasonably close to the value I measured in air. I still have no way to accurately position the sensor way down in the water like that, and the remaining plants can shade the sensor enough to drop it to a 20 reading with just a small movement in either direction. Clearly one would need a tank without plants to measure the actual light intensity unaffected by the plants. I see no way to determine the distribution of intensity as I did in air.